You can open it with NSImage's open image with file init method, then create an NSBitmapImageRep from the TIFFRepresentation of the image. After that, you can get the NSData for various image formats and save them to the disk. (look at the documentation for the exact method names)

Edit: basically exactly what you did with your GlyphTool, but first load the image using NSImage's initWithContentsOfFile method.